At five minutes to five yesterday afternoon, Tehran time, Iranian television viewers finally got the channel they have been asking for. It delivered both national and international news in a snappy, professional style. The first item was about Gaza. The channel also reported the results of a specially commissioned opinion poll which suggests that 94% of Iranians believe their country is entitled to develop civil nuclear power but only 50% are comfortable with the idea of the Islamic Republic having nuclear weapons. Then there was an interactive programme called Your Turn, with people from inside and outside the country ringing and texting in to discuss Iranians' perceptions of themselves and the world's view of Iran.

The only odd thing about this programme, made by Persians, for Persians, in Persian, is that it was broadcast from a BBC studio in the heart of London, and paid for by the British taxpayer. The launch of BBC Persian TV is one of the most unambiguously positive developments I have seen in a long time, and worth every penny of its £15m annual budget (the price of about one bolt on a Trident missile). It responds to repeated demands from Iranians themselves for news they can trust, in a society confused by both organised lying and spontaneous conspiracy theories. It links to the existing BBC Persian radio service, which has been going since 1940, and to an active and interactive BBC Persian website (bbcpersian.com). It draws not just on the worldwide newsgathering resources of the whole BBC but also on its own Persian-speaking correspondents in Washington, Jerusalem, Beirut, Islamabad, Istanbul, Dushanbe and Kabul. (The aim is to reach Persian-speakers in Afghanistan and Tajikistan as well as Iran.)

There are many difficulties, starting with the fact that the Iranian authorities are not allowing BBC Persian to do its own television reporting from Iran. When I visited the studio earlier this week, senior editorial staff suggested that getting and checking the stories is not a problem, since they already have many good news sources there, but doing television without your own fresh pictures is tough. The Iranian authorities have already, predictably, denounced the channel as an instrument of "espionage and psychological warfare", so there's a danger that they will try to intimidate some of its collaborators inside Iran. And a political test looms: in the runup to this summer's presidential elections in Iran, can the channel both get the real story and maintain BBC standards of accuracy, fairness and impartiality?

The claim that this is a nefarious British government plot plays into a rich seam of popular paranoia about Britain. The phenomenon is known to educated Iranians as "Uncle Napoleonism", after the hero of a popular comic novel who is convinced that the British are always pulling the strings behind anything that happens in Iran. In the past, there was often a grain of truth in that suspicion, from the 19th century rivalry with Russia to the toppling of the country's secular leftist leader Muhammad Mossadeq in the early 1950s, but nowadays the influence of perfidious Albion is 95% myth to 5% reality. A British diplomat of my acquaintance likes to joke that Iran is the last place in the world where Britain is still a superpower.

It's not just conspiracy-minded Iranians who find it hard to believe that a television channel funded by the British taxpayer, via the British Foreign Office, can be genuinely independent. He who pays the piper calls the tune. But the BBC World Service, which oversees the Persian channel, has built up a strong track record of defying that logic, especially in recent decades. The current head of the World Service, Nigel Chapman, says he has never been rung up by anyone in government trying to violate the service's clearly codified editorial independence - and if he did receive such a call, his answer would be short and possibly unprintable.

A greater danger is that the service could fall prey to conflicts between Iranian factional tendencies, of the kind that are often magnified in exile, or be seen to be backing a particular opposition group or party line. The BBC Persian journalists I spoke to are determined to avoid this danger.

Instead, Persian-language viewers should be offered something they've never experienced before: a sustained attempt to give a fair reflection of their reality, including the viewers' own feedback, both on air and online. The channel's editor for user-generated content, Sina Motalebi, was himself a blogger in Iran - and in 2003 spent three weeks in solitary confinement for his pains. He spoke to me with passion about the responsible broadcasting, free equally from Iranian and British state control, of important debates in Iran's hyper-active blogosphere. So this is not just a window on to the world for Iranians; it's a window onto themselves.

If the Persian service succeeds in living up to these high ideals, its potential is immense. The only serious competition at the moment is the Washington-based Voice of America Persian TV, which American and Iranian experts on international broadcasting tell me has been a textbook example of how not to do these things, being at once transparently a mouthpiece for the Bush administration, the tendentious voice of particular exile factions, and dull.

BBC Persian aims to have some eight to 10 million viewers in Iran within three years. Its editors make the interesting observation that listening to shortwave radio in Iran has mainly been the private province of men. Now they hope that more women, and whole families, can become part of the conversation, as the BBC enters the living room where the television sits.

This is a long-term journalistic project, not a short-term political one. But the cumulative political impact of offering quality international and national news, analysis and features (including documentaries made by independent filmmakers in Iran), as well as a place for Iranians to listen to each other, can't be overestimated. Abbas Milani, a leading specialist on Iran based at Stanford University, goes so far as to say that "if this had existed 10 years ago, Iran might be a different place today". In 10 years' time, it may yet be.

For, serendipitously, this channel launches at the very moment when the United States is about to change its self-defeating policy of non-engagement with Iran. The nature of the "engagement" recently promised by Hillary Clinton, the next secretary of state, on behalf of president-elect Barack Obama, has yet to be seen. It won't be easy, given that the Iranian nuclear programme has advanced so far on Bush's watch, and Iran's hand has been strengthened by the Iraq war; that the Iranian regime will be loath to abandon the systemic anti-Americanism that has sustained it ever since the Islamic revolution in 1979; and that both sides have had virtually no official presence in the other's country for 30 years. But at least now, if Obama and Clinton want to find out what's really happening in Iran, and if supreme leader Ayatollah Ali Khamenei and president Mahmoud Ahmadinejad want to find out what's behind American foreign policy, they can all watch the BBC.